VIII THE PTERIDOPHYTA—OPHIOGLOSSACEyE 221 



Other classes. A marked characteristic of all Ferns is the 

 large size of the leaves, which are also extremely complicated 

 in form in many of them. In a few of these the leaves are 

 simple, i.e. Ophioglossuin, Vittaria, Pilulai-ui, but more commonly 

 they are pinnately compound and sometimes of enormous size. 

 The stem varies a good deal in form and may be very short 

 and completely subterranean, as in species of Ophioglossmn and 

 Botrychiujn, or it may be a creeping rhizome, or in some of the 

 large tropical Ferns it is upright, and grows to a height of 8 

 to 10 metres, or even more. 



While some forms of the Ferns are found adapted to almost 

 all situations, most of them are moisture -loving plants, and 

 reach their greatest development in the damp forests of the 

 tropics. A few, e.g. Ceratopteris., Azolla, are genuine aquatics, 

 and still others, e.g. species of Gyvinogramme, live where they 

 become absolutely dried up for several months each year. 

 These latter will quickly revive, however, as soon as placed in 

 water, and begin to grow at once. In the tropical and semi- 

 tropical regions many Ferns are epiphytes, and form a most 

 striking feature of the forest vegetation. With few exceptions 

 the sporophyte is long-lived, but a few species are annual, e.g. 

 Pilularia Americana, Marsilia vestita, and depend entirely 

 upon the spores for carrying the plant through from one season 

 to another. The sporophyte may give rise to others by simply 

 branching in the ordinary way, or special buds may be developed 

 either from the stem or upon the leaves {Cystoptej'is bulb if era). 



Besides the normal production of the gametophyte from 

 the spore, it may arise in various ways directly from the 

 sporophyte (apospory) ; and conversely the latter may develop 

 as a bud from the gametophyte without the intervention of the 

 sexual organs (apogamy). 



The Filicineae include both eusporangiate and leptospor- 

 angiate forms, — indeed the latter occur only here. The former 

 comprise the homosporous orders, Ophioglossacese and Maratti- 

 aceae, and the heterosporous order Isoetaceae, whose systematic 

 position, however, it must be said is still doubtful. The 

 Leptosporangiatae include the single great homosporous order 

 Filices, and the two heterosporous families, closely related to 

 it, the Salviniaceae and the Marsiliacea^. These are usually 

 classed together as a distinct order, the Hydropterides or 

 Rhizocarpeae. 



